This invention relates to acoustic sensing arrangements for use in underwater applications.
As the noise to be detected (e.g. noise from submarines) by such sensing arrangements becomes quieter, the acoustic noise produced by the detection device becomes more significant. This means that the acoustic sensor array must be well isolated from the motion of the surface water, and that the noise generated by water flow over the sensors and array structure must be reduced to a minimum. As isolation can never be 100% and, due to the presence of shear currents, there will always be some water flow over the sensor(s), some means is required to eliminate the effects of these movements. In addition, it is desirable that a single acoustic sensor assembly can be used to determine accurately the bearing of an acoustic source.
A sonobuoy consists of an acoustic sensor assembly suspended by a cable below a radio transmitter which floats on the sea surface. To achieve isolation of the acoustic sensor from the wave-induced motion of the floating radio transmitter unit known sonobuoys use a damped spring-mass system (i.e. decoupling system) comprising an elastic section (i.e. compliance) in the suspension cable and a highdrag sea anchor (i.e. drogue) at, or near, the acoustic sensor, to provide a large virtual mass. The compliance normally consists of a long section of high elasticity rubber to give low stiffness and the sea anchor may be a large diameter horizontal fabric disc with or without vertical vanes and erected by a spring ring or other collapsible framework. The vertical isolation is sometimes further improved by configuring the flotation unit as a spar buoy so that it does not follow the full motion of the sea surface. However, the spar buoy has the disadvantage that, in high sea states, it is more susceptible to washover and consequent loss of r.f. transmission. The effect of the decoupling is to reduce the vertical movement of the acoustic sensor to about one twentieth of the sea surface motion. The vertical components of the drogue (if fitted) reduce the horizontal flow due to shear currents over the sensor. Although the water flow over the acoustic sensor is much reduced by the decoupling system, there is still some cyclic vertical flow, and uni-directional horizontal flow. This generates noise due to vortex shedding etc.